Superconducting magnets for generating very powerful magnetic fields are well known. Such superconducting magnets find application in the suspension and propulsion systems of maglev (magnetic levitation) transportation systems, among other things. Such superconducting magnets are utilized in both electrodynamic suspension (EDS) and electromagnetic suspension (EMS) magnetic levitation systems. Superconducting magnets are extremely efficient when utilized in such applications since no energy is lost to resistive or joule heating along the superconducting current path.
However, every superconducting material has an associated critical temperature T.sub.c above which the material ceases to function as a superconductor and becomes a normal conductor within which joule heating occurs. This effect is known in the art as quenching.
As long as the region where such quenching occurs is small enough or receives sufficient cooling, then the heat produced by such normal resistance will dissipate and the material will generally resume a superconducting state.
However, if the region of joule heating is sufficiently large that the system's ability to dissipate heat is overwhelmed, then the area of loss of superconductivity will be maintained and will tend to grow in size. Such propagation of the region of normal conductivity typically results in catastrophic failure of the superconducting magnet, thereby potentially causing damage to the associated apparatus. This effect is particularly undesirable in maglev applications.
For example, if such catastrophic failure of the superconducting magnet were to occur when a maglev vehicle is traveling at a high speed, then undesirable contact of the vehicle with the track may result, potentially resulting in serious damage to the vehicle and/or the track.
When it is known that quenching has occurred, then the superconducting magnet can be shut down in a controlled and safe manner, thereby avoiding such damage. Thus, it is desirable to quickly and accurately identify the loss of superconductivity within the coils of superconducting magnets.
Various attempts at detecting the loss of superconductivity within a superconducting magnet have been tried in the prior art. Although such prior art detection schemes have proven generally suitable for their intended purposes, none address the particular problems of detecting loss of superconductivity within the coils of a superconducting magnet utilized in maglev applications.
The best of such prior art methodologies for detecting quenching of the coils of superconducting magnets measure a resistive voltage signal or phase change within the superconductive coil. Moreover, such methodologies are not feasible in maglev applications due to the extremely wide bandwidth of AC and DC field fluctuations associated therewith. For example, the DC field for providing base excitation in such maglev applications is approximately 1 Hz, which results from compensating for slow changes in the maglev vehicle loading; 10-20 Hz oscillations due to uneven air gap length between the iron poles and the rail; and up to 1.5 kHz oscillations due to air gap permeance variations caused by slots in the iron rail. As those skilled in the art will appreciate, a broad spectrum of field fluctuations makes it difficult to utilize quench detection techniques based upon voltage changes within the coil of the superconducting magnet. It is very difficult to identify those voltage changes which are caused solely by a temperature rise, as opposed to those having a contribution from such field fluctuations.
In view of the foregoing, it is desirable to provide means for reliably and accurately detecting quenching of the coil of a superconducting magnet utilized in maglev applications in a manner which is sufficiently timely to facilitate controlled shut down thereof.